Filters are well known devices for removing contaminants from a flow of fluid or gas. Filters are sometimes included in air compressor systems supplying compressed air to paint spray guns. Such systems typically include an air compressor, a compressed air storage tank, a pressure regulator, valves, and a flexible hose to convey the compressed air to the spray gun. In a commercial painting operation, a central compressor can supply a number of paint spray guns through an extensive compressed air distribution system. Air compressors are sometimes known to introduce oil and water droplets into the air as it is compressed. Furthermore, as the compressed air passes from the compressor through air lines and hoses to the spray gun, dirt, rust and other debris can become entrained in the air flow. The operation of the spray gun will mix these contaminants with the paint and thereby apply an inferior coat of paint. Furthermore, water and oil have been known to condense within air lines during equipment downtime. Upon restarting painting operations, such condensate can be expelled through the spray gun. Therefore, air filters are often included in compressor systems to remove these contaminants from the compressed air. Such air filters are typically inserted into the compressed air supply line in close proximity to the spray gun. As an alternative location, a small air filter can be mounted directly upon the gun.
Compressed air filters typically include a cylindrical rigid housing having an inlet port at one end for receiving unfiltered compressed air and an outlet port at the other end for discharging filtered air. A cylindrical filter element is disposed within the housing between the inlet and outlet ports. The filter element is typically formed from a porous paper or a cloth material or an open celled foam material. The structure of the filter causes an axial flow of air through the filter element. The filter element absorbs water and oil droplets and entraps any dirt, rust or other debris which might be present.
The filter element presents a resistance to the flow of compressed air through the filter, causing a pressure drop across the filter. As the filter element accumulates contaminants, the flow resistance and accompanying pressure drop increase. Eventually, the filter element can become completely plugged, blocking the flow of air through the filter to the spray gun. If the filter receives a slug of water from the air line, the filter media may attempt to expand. In prior art filters, the filter media is confined. When the filter media absorbs water, the resulting expansion closes the air passages and the pressure drop across the filter increases. Thus, the filters have a finite life and must be periodically replaced.
Since the pressure supplied to the filter is held relatively constant by a pressure regulator, any increase in pressure drop across the filter results in lower pressure available to operate the spray gun. Even with periodic filter replacement, the decline of air pressure available to the spray gun during the later portion of the filter life can adversely affect spray gun performance. Thus, there is a need for an air filter that maintains a low pressure drop across its filter element throughout the filter life.